Dynamic impact wear of TiC_xN_y and Ti-DLC composite coatings

Dynamic impact wear conditions, i.e. contacts in the presence of high cyclic local loads, provide a very demanding wear situation, which occurs in a number of applications, e.g. in interrupted machining or impact contacts between machine parts. Thus, protective coatings for these conditions and methods for their evaluation are currently being pursued by several laboratories. Magnetron sputtered composite coatings of TiC_xN_y and Ti-containing amorphous diamond-like coatings (Ti_x%-DLC) of different compositions offer a promising solution and are evaluated here in terms of their microhardness, scratch adhesion and abrasive wheel performance, and with a recently developed cyclic impact test. In order to provide a severe test for the coatings, a relatively soft substrate material (316 stainless steel) was utilised, which ensured that a degree of plastic deformation occurred beneath the coatings, thereby giving a qualitative indication of the adhesive and cohesive properties of the coatings. SEM and optical inspections of wear regions allowed the development of a model to describe hard coating degradation on soft substrates under periodic impact loads. A central zone of cohesive failure, an intermediate zone of cohesive-adhesive failure and a peripheral zone with circular cracks were distinguished in the coatings tested. A TiC_0.75N_0.25 coating was found to have the best resistance to this kind of dynamic wear. The incorporation of supporting layers for (Ti_x%-DLC) coatings was demonstrated to improve the effective adhesion.